I know no one else responded because he told me no one else responded.
As for hiding the key: I thought initially of trying it, but then thought better of it. Too risky. We have people coming through this neighborhood casing the houses (a thing I have personally observed, and which we are cautioned by the police to be aware of). There isn't a good place to hide it. I don't even have a mat in front of the house (wouldn't want to leave a key under the mat if I did, as that is so cliche).
Anyway, the guest arrived. I gave him his key, some clean towels, and the password to our WiFi, and I made him a pot of fresh coffee. Then I left to meet my friend for our longstanding plans.
What I was reacting to before with the "I guess I should never ask for anything" was the number of people who told me I was flat out wrong to inquire about petsitting (which, BTW, has already been resolved. Long resolved). I can't understand why, if I am doing a giant, huge favor for someone, I can't inquire about the possibility of their doing me a favor in return. Evidently that was a giant faux pas on my part. I dropped it immediately as soon as I learned that he didn't want to do it.
I live in an upscale neighborhood in one of the top tourist destination cities in the world, convenient to mass transit, a beautiful, quiet house. So when I think of offering someone to petsit, it's not as if I am asking them to stay in a hovel or an undesirable location. Once someone drove from another state to stay here and was thrilled to get time in SF for free in exchange for feeding a few pets. No one is obligated to petsit for me, but if someone is excited about traveling to my city and excited about staying in my house, it crossed my mind that it might suit both of us for him to extend his stay a few days. It's not bait and switch, because he still gets what he wants (staying in my house for free) without having to do anything in return for me.